You see, the faster you move - the greater your motion is through space - the slower your motion becomes through time. But they exist all the same, however subtly. If you’re moving at just a few meters-per-hour, or a few kilometers-per-hour, or even a few kilometers-per-second, like the Earth does orbiting the Sun, you probably won’t even notice the barriers that exist to moving at an infinite speed. You might think that by expending more and more energy, you can make yourself move faster… and while this is true, it’s only true up to a point. No matter where you are or what you are, there’s an absolute limit to how quickly you can move through space. Public domain timelapse photo by flickr user comedynose (Pete), illustrating fast, relativistic. Depending on how you view it, the expansion of the Universe itself isn't bound by the speed of light at all. In the expanding Universe - in curved spacetime in general - the rules are very different. In fact, it still can't even reach the speed of light itself! But those rules only apply, strictly, to particles at the same location as one another in spacetime. Even more surprising and counterintuitive is this: if a particle moving close to the speed of light fires out another particle moving close to the speed of light, it doesn't move at almost twice the speed of light. If you're a massless particle, you must travel at that speed, and if you have a non-zero mass, it's impossible for you to attain that speed, no matter how much energy you pump into it. One of Einstein's most famous fundamental laws is that nothing in the Universe can travel faster than the speed of light in a vacuum.
Image credit: Pablo Carlos Budassi (Unmismoobjetivo) under a c.c.a.-s.a.-3.0 license. structure and the hot, dense plasma of the Big Bang at the outskirts. Artist's logarithmic scale conception of the observable universe.
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